Mature mammalian T lymphocytes are not postmitotic, but rather can cycle indefinitely between states of rapid cell division and extended quiescence. Each phase of proliferation is apparently triggered by a single polypeptide hormone, interleukin 2 (IL-2). The resting state is associated with the activation of an endogenous endonuclease that introduces nicks in chromatin DNA. Both the specificity of the growth signal and the reversibility of the resting state make this system interesting. During their maturation in the thymus, T-cell precursors are subject to another pattern of growth control. After rapid proliferation, the vast majority arrest their growth, become inert, and die. The mechanisms involved in their death may be unique to immature cells or related to the mechanisms that mediate reversible growth arrest in mature T cells. Our proposal concerns three aspects of growth regulation. One is the molecular basis of IL-2 expression. We will clone the mouse IL-2 genomic DNA and introduce it into lymphoid and nonlymphoid cells to identify sequences necessary for its correct transcription. Another area is the relationship between the normal growth mediator IL-2 and malignant growth. We plan to link an expressible copy of the IL-2 gene to a constitutive promoter and use it to transform cells that are permanently responsive to IL-2, then test these cells for tumorigenicity in vivo. The third and most complex issue is the basis of programmed thymocyte death and its relationship to the mechanism of reversible growth arrest. We will examine interactions between endonucleolytic chromatin nicking, perhaps common to all resting T cells, and the unbalanced nucleotide metabolism and terminal deoxynucleotidyl transferase (TdT) expression that are charcteristic of doomed thymocytes. The cloned TdT gene will be introduced into mature T-cell lines to ask whether TdT activity potentiates DNA damage and/or interacts with drugs that perturb deoxynucleotide pools. The results will test two models for thymocyte death, either linking it to or dissociating it clearly from the growth arrest in mature cells. (HF)